Here's one datum point to consider.
Some may recall I posted in another thread that, at about 500 clicks, I investigated for dust, found some and cleaned same (wet and dry). I had also mentioned that the dust I did find had only intruded into a short series I had shot (a jetliner at altitude in a clear sky at sunset). Those spots were not only evident but childs play to remove in PP. Let me add that the camera is set to clean the sensor at on and off - this has been my standard choice with the D300 and D90.
That cleaning seemed to do the trick. Now, with the Nikon service advisory I decided, at 2500 clicks to go back and investigate. I can say quite honestly that I have not noticed dust in any of the intervening photographs. There have been a lot of blue skies and a lot of overcast skies as well as copious amounts of snow on the ground but, shooting, as I do, at apertures typically no smaller than f/13, I simply have not noticed anything.
Shooting a dust photo today I found a few, mostly in the margins, i.e., the ~100 pixels next to the edges. (iso LO1, f32, 1/15, 70/300 at 300mm, shot straight up, panning across a very homogeneous overcast - the histogram was a spike in the middle of the right half of the histogram) OK, says I, I shall blow the sensor off and that, me hearties, will be that. It wasn't. Curiously, there was no great difference between the before blowing and after photos. Some of the spots persisted and some seem to have decided to take up residence in different neighborhoods, again in the margins, not in the center. Mixed progress, but not a great deal, cumulatively.
On the other hand, three discrete passes with the Arctic Butterfly cleaned the sensor right up, even the last rows nearest the earth (on the sensor), i.e., the top ~100 rows in the sky. No wet cleaning involved this time.
This D600 (Ser. 306xxxx) does not seem to need to go back to Nikon. YMMV